Digital data is often stored and retrieved from memory. Memory can be manufactured in silicon. Semiconductor materials etched onto the silicon allow many transistors to be implemented in silicon to produce high density memories. The transistors may be configured to form logic gates, inverters, and other functions used to implement the memory.
To increase memory density, a single memory cell is now designed to contain two or more bits of data. For example, a floating gate transistor acting as a capacitor may be used to store a cell voltage that represents two bits of data. The cell voltage may be programmed by injecting electrons onto the transistor floating gate. Storing two or more bits in a memory cell increases the density of data that may be stored in the memory cell.
However, reading two or more bits of data is more difficult than reading one bit of data. For example, the memory cell may store the bit values of 00 when the cell voltage is a first level. The bit values represented in the memory cell may be 01, 10, and 11, respectively, when the cell voltage is a second, third, or fourth voltage level. To determine what bit values the cell voltage represents now requires four or more voltage comparisons because the cell voltage now represents one of four voltage levels. A better way to access memory is desired.